Preheat oven according to cake mix package directions and grease a baking pan.

In a large bowl, combine cake mix and strawberry gelatin.

Add eggs, melted butter, milk, and vanilla extract. Mix until smooth.

Gently fold in mashed strawberries until evenly incorporated.

Check batter consistency and adjust with a small amount of milk if needed.

Pour batter into prepared pan and spread evenly.

Bake according to package directions, checking a few minutes early for doneness.

Remove from oven and let cool completely before frosting or serving.